Generally, an electrochromic device is capable of modulating the optical properties (transmittance and/or reflectance and/or absorbance) when an electric field is applied thereacross. More specifically, it comprises at least one active electrochromic electrode capable of ensuring a reversible change of optical state on application of an electric charge.
A conventional electrochromic device generally comprises a support successively supporting (FIG. 6):
a first current collector (4);
a first electrochromic electrode (11);
an electrolyte (12);
a second electrochromic electrode (13); and
a second current collector (6).
At least one of the electrochromic electrodes is optically active. It enables to modulate the optical properties of an electromagnetic radiation applied to the device. Such an optical state switching is obtained by application of an electric field across the electrochromic device.
The used electrochromic materials may be organic, inorganic, or hybrid. Their nature enables to pass from one optical state to another by insertion or deinsertion of cations. Such a cation transport is easier when the cations are small; they generally are protons (H+) or lithium ions (Li+).
As already indicated, the change of optical state is triggered by application of an electric field across the electrochromic device; this property enables to modulate the reflection and the transmission of an electromagnetic radiation having a wavelength which may vary according to the range of application. Thus, the visible UV range may correspond to applications of electrochromic window type, while the infrared range may correspond to an application of heat management type.
As already indicated, electrochromic devices comprise two electrochromic electrodes. Typically, the second electrochromic electrode enables to store cations. It may be transparent (optically passive), whatever the cation flow. It may also act as a complementary electrode and have an optical state (transparent, colored . . . ) identical to that of the first electrode but with an inverse cation flow.
Electrochromic devices having this second type of configuration (complementary electrode) are generally preferred, given that they improve the optical perception of the change of optical state (contrast). For example, the first electrode may be made of a WO3 material while the second electrode may be made of NiO. This couple of materials allows the following electrochemical reactions:WO3+xLi+ (transparent)←→LixWO3 (colored)LixNiO (transparent)←→NiO+xLi+ (colored)
Generally, electrochromic materials allow a reversible bistable change of optical state when a voltage is applied across the electrochromic device. In other words, such materials have two steady states, and require no power input to be maintained in one of the two states.
The performance of electrochromic devices is particularly assessed by means of the following indicators:                contrast: the difference between the maximum and the minimum of the optical response of the device. The higher the contrast, the more effective the device is considered.        optical density: the quantity of charges to be brought to the system to switch, that is, to pass from the minimum state to the maximum state or conversely. For an equal charge, the greater the optical transformation, the more effective the device is considered.        switching time: the time necessary for the device to ensure the passing from one optical state to another, which is fixed for a given contrast. The shorter the switching time, the more effective the device is considered.        
All-solid electrochromic devices operating by insertion of cations (Li+ for example) within inorganic materials may have relatively long switching times. Indeed, Li+ cations are less mobile than protons. Further, an all-solid electrolyte has a lower ion conductivity than a liquid electrolyte. Accordingly, the cation migration kinetics is slower, which lengthens the switching time.
As an example, document U.S. Pat. No. 7,265,890 describes an all-solid inorganic electrochromic device operating by insertion of Li+ cations in the infrared range. It comprises a first electrochromic electrode, an electrolyte, and a second transparent electrochromic electrode behaving as an Li+ cation storage electrode.
Such an electrochromic device is typical of prior art configurations. The second electrochromic electrode is a transparent cation storage electrode, which is thus optically passive. Generally, it has a cation storage capacity larger than that of the first electrochromic electrode forming the active electrode.
Typically, the quantity of cations (Li+ for example) injected into the storage electrode corresponds to the maximum capacity of cations which can be reversibly inserted into the available thickness and de-inserted therefrom.
The saturation of the storage electrode enables to compensate for a possible loss of cations during the cycling (insertion irreversibility). It further enables to improve the chemical stability of the device by anticipating the possible oxidation of part of the cations, which might cause a failure of the device.
Anyhow, prior art electrochromic devices are generally not compatible with a use in extreme temperature conditions, given that the temperature of use has an effect on the contrast and the switching time.
The present invention provides solving this technical problem due to the use of electrochromic devices at negative temperatures.